1. Field of the Invention
One embodiment of the present invention relates to an organic compound. One embodiment of the present invention further relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each of which uses the organic compound.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Such a light-emitting element is a self-luminous element and has advantages over liquid crystal displays, such as high visibility of pixels and no need of a backlight; thus, the light-emitting element is thought to be suitable as a flat panel display element. Besides, the light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed.
Furthermore, since the light-emitting element can be formed in a film form, it is possible to provide planar light emission; thus, a large-area element utilizing planar light emission can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to lighting and the like.
Light-emitting elements utilizing electroluminescence can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, application of voltage to the light-emitting element causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus current flows. The injected electrons and holes then lead the organic compound to its excited state, so that light emission is obtained from the excited organic compound.
The excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and light emission from the triplet excited state (T*) is called phosphorescence.
In improving element characteristics of the light-emitting element, there are many problems which depend on a substance, and in order to solve the problems, improvement of an element structure, development of a substance, and the like have been carried out. For example, Patent Document 1 discloses a carbazole derivative having a high hole-transport property as an organic compound that can be used for forming a light-emitting element with high emission efficiency.